[feiiiiii5]

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Describe your changes

Partial[Model].model_from_chunks() loses nested model instances for any field declared with a validation alias.

_build_partial_object() iterated the parsed JSON keys and used them twice: as the lookup key into model.model_fields, and as the key of the dict handed to model.model_construct(). model_fields is keyed by the Python attribute name, while the streamed JSON is keyed by the alias that model_json_schema() advertises, so both uses miss for an aliased field. The consequence is two-fold:

• the field type is never found, so a complete nested object is stored as a raw dict instead of being validated;
• the value is stored under the alias key, so model_construct() populates __dict__ with loc and the declared attribute location falls through to the "missing fields" branch and is replaced by an empty model.

Callers therefore get obj.location == {} plus a stray obj.loc dict, and obj.location.city raises AttributeError. Same for aliased list[Model] and Optional[Model] fields.

The patch resolves the parsed key to the attribute name once per field (canonical_field_name, covering alias, validation_alias and AliasChoices), keeps the raw key for the completeness tracker because the tracker works on the JSON text, and stores under the attribute name. Models without aliases take the key in model.model_fields fast path and behave exactly as before.

Repro (no API key needed, this is what the provider streaming handlers call):

class Location(BaseModel):
    city: str
    lat: float

class Aliased(BaseModel):
    answer: str
    location: Location = Field(alias="loc")

chunk = '{"answer": "hi", "loc": {"city": "NYC", "lat": 1.5}, "tail": '
print(list(Partial[Aliased].model_from_chunks(iter([chunk])))[-1].__dict__)
# before: {'answer': 'hi', 'loc': {'city': 'NYC', 'lat': 1.5}, 'location': Location(city='', lat=0.0)}
# after:  {'answer': 'hi', 'location': Location(city='NYC', lat=1.5)}

Issue ticket number and link

No existing issue or open PR covers this — I searched repo:567-labs/instructor for alias, partial alias and model_from_chunks across issues and PRs before opening. #2600 changed the same function for nullable list fields, which is the behaviour this extends to aliased fields.

Checklist before requesting a review

• I have performed a self-review of my code
• If it is a core feature, I have added thorough tests.
• If it is a core feature, I have added documentation. — no user-facing documentation describes alias handling in partial streaming, so there is nothing to correct; say the word and I will add a note to docs/concepts/streaming.md.

How this was verified

details

Environment: python 3.11, PYTHONPATH pointing at the checked-out tree, instructor.__file__ confirmed to resolve inside the tree, -p no:opik to keep conda's unrelated plugin from aborting collection.

Base control (e12f8b49203b0c1f253d27c1e709d0a09b9fc5a8, instructor/v2/dsl/partial.py restored from that commit, git status for the file empty):

$ python -m pytest tests/dsl/test_partial_aliases.py -p no:opik -q
2 failed, 1 passed in 0.37s

The passing case is the deliberate non-alias control, which shows the harness measures the real path rather than failing for an unrelated reason.

Same file, same command, with the patch applied:

$ python -m pytest tests/dsl/test_partial_aliases.py -p no:opik -q
3 passed in 0.25s

No regression in the surrounding suites:

$ python -m pytest tests/dsl tests/coverage/test_dsl_partial_coverage.py tests/core/test_schema.py -p no:opik -q
167 passed, 2 skipped

Static checks, both clean:

$ python -m ruff check instructor/v2/dsl/partial.py tests/dsl/test_partial_aliases.py   # All checks passed!
$ python -m ruff format --check instructor/v2/dsl/partial.py tests/dsl/test_partial_aliases.py   # 2 files already formatted
$ python -m ty check instructor/v2/dsl/partial.py   # All checks passed!

Per CLAUDE.md a behavior change needs a [Unreleased] entry in CHANGELOG.md; that entry links the PR number, so it follows in a second commit as soon as this PR is open.

Known limitations

details

• validation_alias=AliasPath(...) is not resolved in partial snapshots: _alias_keys() only collects string aliases and AliasChoices, and an alias path would need its nested value lifted while the object is still incomplete. Covered here only as a documented scope boundary; the reproduction lives in the review thread. String aliases (the plain alias= case that dropped fields before this PR) are covered.

[feiiiiii5]

Hi — anything you'd like changed before this can go in? Both GitHub Actions workflows on the fork PR are parked at action_required, so the suite has not actually run yet; locally pytest tests/ and ruff are clean on the branch, and it is rebased on the current main (head 2b02833).

The change resolves partial-streaming fields that arrive under a validation alias rather than dropping them, and the regression test streams a chunk that carries only the alias and asserts the resolved field reaches the model. Happy to switch to a different shape if you would rather it error than resolve.

[feiiiiii5]

Correction to the line about a clean local pytest/ruff run: drop it. I did not run them for this PR and I cannot point to a recorded result, so that part is unverified.

What I can state is the rest: both workflows are parked at action_required, and the branch sits on the current main at 2b02833. If you would rather not approve the workflows, say so and I will run the suite here and report the output.

[lj1166]

The string-alias coverage here fixes the plain alias= case, but AliasPath remains unresolved: _alias_keys() discards non-string path aliases, so canonical_field_name() never maps the parsed outer key.

Minimal reproduction on current main (Python 3.12, Pydantic 2.12.5):

from pydantic import AliasPath, BaseModel, Field
from instructor.dsl.partial import Partial

class Inner(BaseModel):
    value: int

class Outer(BaseModel):
    payload: Inner = Field(validation_alias=AliasPath("container", "payload"))

snapshot = list(Partial[Outer].model_from_chunks(['{"container":{"payload":{"value":1}']))[-1]
assert isinstance(snapshot.payload, Inner)  # current: dict

Outer.model_validate({"container": {"payload": {"value": 1}}}) produces Inner(value=1), so the streaming snapshot diverges before the root closes. Could the PR either add an AliasPath regression or explicitly scope it out? The current helper's isinstance(key, str) guard means the test would remain red after this patch.

[feiiiiii5]

Good catch — that is a real limitation, but it is a different mechanism from what this PR fixes, not a missing branch in the alias-key set.

_alias_keys() only collects string aliases and AliasChoices entries. An AliasPath maps a nested input ({"container": {"payload": ...}}) to a field, so supporting it means lifting the value along the path while the surrounding object is still incomplete — the snapshot builder currently validates the top-level dict, where there is no payload key to resolve. Doing that properly is its own change, and I would rather not grow this PR after review without a maintainer asking for it.

I have added an explicit "Known limitations" note to the PR description pointing at AliasPath (with the reproduction from this thread) so the scope is documented rather than implied. If you or a maintainer want AliasPath folded in here, say so and I will add the path-lifting plus your regression; otherwise it belongs in a follow-up issue.

[lj1166]

Thanks for clarifying the boundary. I split the AliasPath behavior into #2872 and opened #2873 with sync, async, and indexed-path regression coverage, so #2668 can stay focused on its string-alias fix.

[feiiiiii5]

Thanks for splitting the AliasPath behavior out — #2668 stays scoped to string aliases (and now resolves AliasChoices string choices without tripping over unhashable alias objects; AliasPath is documented as a known limitation and no longer crashes). I'll leave #2872/#2873 to the reviewers you mentioned.